The present invention claims priority from Japanese Patent Application No. 2000-034133 filed Feb. 10, 2000, the contents of which are incorporated herein by reference.
1. Field of the Invention
The present invention relates to a radio communication system for use in a place of business and, particularly, to a radio communication system for a place of business, which allows a call to be saved when a radio terminal is moved out of a service area of a radio base station thereof.
2. Description of Related Art
A private telephone service, in which a plurality of base radio stations each having a service area having a diameter in a range from several tens meters to several hundreds meters are arranged outside of a place of business such as factory and a mobile radio terminal is connected to a public telephone network through one of the base radio stations, has been put in practice. Although such private telephone service is similar to the cordless telephone system for home use, it differs from the cordless telephone system in that a telephone number is assigned to not a base radio station corresponding to a subscriber terminal in the cordless telephone system but a radio terminal corresponding to an extension terminal in the same system. Since the service area of the private telephone service is narrow and a usable range of the service can be defined, the private telephone service is suitable for use as a radio communication system in a private area. However, even if it is used in a private area, many locations exist outside the service area, in which communication between the base radio station and the radio terminal becomes impossible due to attenuation or cut-off of electromagnetic wave, so that a call from the public telephone network to radio terminal in such a location and a communication between the network and the radio terminal become impossible.
In order to allow a call from a public telephone network to a radio terminal located outside a service area, in which communication is possible, many proposals have been made. One of them, which is disclosed in JP H9-307650 A, is an automatic transfer system, in which a radio terminal is registered as an extension of a private branch exchange or a terminal of a key station, so that a call from a telephone to the radio terminal can be received by the same radio terminal by using a public telephone number assigned thereto. Another of them, which is disclosed in JP H8-019044 A, is a private automatic exchange system, in which a call is transmitted to a called side by utilizing a public telephone network. Another of them, which is disclosed in JP H10-136450 A, is a private radio communication system, in which a calling side can be easily confirmed by a called side in a case where the called side is out of a service area when the call is received thereby.
In such conventional automatic transfer system, etc., there is a problem that, when a called radio terminal moves to a location outside a service area of a radio base station during a communication with a calling side fixed terminal, the calling side fixed terminal can not continue the communication since a call connection circuit is broken off. Further, in the radio communication system, there is another problem that, when the receiving state of electromagnetic wave in a radio base station is not stable, a discontinuity of communication tends to occur, resulting in an unstable communication state.
In order to continue the communication when a call connection, which is once broken off during a communication between the radio terminal and the other side fixed terminal due to movement of the radio terminal out of the service area, is restored, it is necessary to make a call from the radio terminal or the other side fixed terminal again. However, in order to make the call connection between the radio terminal outside the service area and the other side fixed terminal possible, it is necessary to construct the previously mentioned special system, since there has been no simple system capable of communicating with the radio terminal.
An object of the present invention is to provide a private radio communication system, a private branch exchange and a radio terminal, with which, when a radio terminal moves out of a service area thereof during a communication between the radio terminal and the other side fixed terminal, a some communication from the other side fixed terminal to the radio terminal can be continued.
Another object of the present invention is to provide a private radio communication system, a private branch exchange and a radio terminal, with which, when the radio terminal moves out of a service area thereof, a call connection of the other side fixed terminal is prevented from being broken off to make a communication of the other side fixed terminal to the radio terminal possible.
A further object of the present invention is to provide a private radio communication system, a private branch exchange and a radio terminal, with which, when the radio terminal moves out of a service area of a radio base station during a communication established between the radio terminal and a fixed terminal, the fixed terminal can be connected to a predetermined fixed terminal device to transfer the communication to the latter fixed terminal device.
In order to achieve the above objects, a private radio communication system according to the present invention, in which a radio terminal is connected to a private branch exchange through a radio base station, is featured by that, when the radio terminal, which is connected to a preliminarily assigned fixed terminal through the radio base station and a private branch exchange, moves out of a service area of the radio base station, the private branch exchange detects a cut-off of a receiving electromagnetic wave from the radio base station and switches the fixed terminal to a predetermined fixed terminal device on the basis of the detection. Further, the private branch exchange is featured by that it holds a connection information between the fixed terminal device and the fixed terminal according to a notice from the radio terminal and the fixed terminal device is a voice mail device or a fixed terminal in a place of business to which a user of the radio terminal belongs.
The private branch exchange according to the present invention, which is used in a private radio communication system for connecting a radio terminal to a fixed terminal through a radio base station, is featured by that the private branch exchange holds a correspondence information between a preliminarily assigned fixed terminal and a preliminarily assigned fixed terminal device and, when the radio terminal, which is connected to the fixed terminal through the radio base station and the private branch exchange, moves out of service areas of all of the radio base stations, switches the fixed terminal to the fixed terminal device. The correspondence information between the fixed terminal and the fixed terminal device is held according to a notice from the radio terminal.
The radio terminal according to the present invention, which is used in a private radio communication system for connecting a radio terminal to a fixed terminal through a radio base station, is featured by that the radio terminal comprises control means, which, when the radio terminal connected to a preliminarily assigned fixed terminal through the radio base station and the private branch exchange moves out of service areas of all of the radio base stations, transmits the correspondence information between the fixed terminal and the fixed terminal device to the private branch exchange such that the fixed terminal is switched to the fixed terminal device, and preliminarily registers the correspondence information in the private branch exchange. Further, the fixed terminal device is a fixed terminal or a voice mail device in a place of business to which a user of the radio terminal belongs.
According to the present invention, when the radio terminal, which is communicating with a fixed terminal through the radio base station, moves out of the service area of the radio base station, the private branch exchange connects the other side fixed terminal to a predetermined fixed terminal device such as a voice mail device to maintain a circuit connection, so that an indirect communication between the radio terminal and the other side fixed terminal becomes possible.